The dhclient.conf file contains configuration information
for
dhclient(8), the Internet Software Consortium DHCP Client.
The dhclient.conf file is a free-form ASCII text file. It
is parsed by
the recursive-descent parser built into dhclient(8). The
file may contain
extra tabs and newlines for formatting purposes. Keywords in the
file are case-insensitive. Comments may be placed anywhere
within the
file (except within quotes). Comments begin with the `#'
character and
end at the end of the line.
The dhclient.conf file can be used to configure the behaviour of the
client in a wide variety of ways: protocol timing, information requested
from the server, information required of the server, defaults to use if
the server does not provide certain information, values with
which to
override information provided by the server, or values to
prepend or append
to information provided by the server. The configuration file can
also be preinitialized with addresses to use on networks
that don't have
DHCP servers.

The timing behaviour of the client need not be configured by
the user.
If no timing configuration is provided by the user, a fairly
reasonable
timing behaviour will be used by default - one which results
in fairly
timely updates without placing an inordinate load on the
server.
The following statements can be used to adjust the timing
behaviour of
the DHCP client if required, however:
timeouttime;
The timeout statement determines the amount of time
that must
pass between the time that the client begins to try
to determine
its address and the time that it decides that it's
not going to
be able to contact a server. By default, this timeout is sixty
seconds. After the timeout has passed, if there are
any static
leases defined in the configuration file, or any
leases remaining
in the lease database that have not yet expired, the
client will
loop through these leases attempting to validate
them, and if it
finds one that appears to be valid, it will use that
lease's address.
If there are no valid static leases or unexpired leases
in the lease database, the client will restart the
protocol after
the defined retry interval.
retrytime;
The retry statement determines the time that must
pass after the
client has determined that there is no DHCP server
present before
it tries again to contact a DHCP server. By default, this is
five minutes.
select-timeouttime;
It is possible (some might say desirable) for there
to be more
than one DHCP server serving any given network. In
this case, it
is possible that a client may be sent more than one
offer in response
to its initial lease discovery message. It
may be that
one of these offers is preferable to the other
(e.g., one offer
may have the address the client previously used, and
the other
may not).
The select-timeout is the time after the client
sends its first
lease discovery request at which it stops waiting
for offers from
servers, assuming that it has received at least one
such offer.
If no offers have been received by the time the
select-timeout
has expired, the client will accept the first offer
that arrives.
By default, the select-timeout is zero seconds -
that is, the
client will take the first offer it sees.
reboottime;
When the client is restarted, it first tries to
reacquire the
last address it had. This is called the INIT-REBOOT
state. If
it is still attached to the same network it was attached to when
it last ran, this is the quickest way to get started. The reboot
statement sets the time that must elapse after the
client first
tries to reacquire its old address before it gives
up and tries
to discover a new address. By default, the reboot
timeout is ten
seconds.
backoff-cutofftime;
The client uses an exponential backoff algorithm
with some randomness,
so that if many clients try to configure
themselves at
the same time, they will not make their requests in
lockstep.
The backoff-cutoff statement determines the maximum
amount of
time that the client is allowed to back off. It defaults to two
minutes.
initial-intervaltime;
The initial-interval statement sets the amount of
time between
the first attempt to reach a server and the second
attempt to
reach a server. Each time a message is sent, the
interval between
messages is incremented by twice the current
interval multiplied
by a random number between zero and one. If
it is
greater than the backoff-cutoff amount, it is set to
that amount.
It defaults to ten seconds.

The DHCP protocol allows the client to request that the
server send it
specific information, and not send it other information that
it is not
prepared to accept. The protocol also allows the client to
reject offers
from servers if they don't contain information the client
needs, or if
the information provided is not satisfactory.
There is a variety of data contained in offers that DHCP
servers send to
DHCP clients. The data that can be specifically requested
is what are
called DHCP Options. DHCP Options are defined in dhcp-options(5).
request [option] [, ... option];
The request statement causes the client to request
that any server
responding to the client send the client its values for the
specified options. Only the option names should be
specified in
the request statement - not option parameters.
require [option] [, ... option];
The require statement lists options that must be
sent in order
for an offer to be accepted. Offers that do not
contain all the
listed options will be ignored.
send { [option declaration] [, ... option declaration] }
The send statement causes the client to send the
specified options
to the server with the specified values.
These are full
option declarations as described in dhcp-options(5).
Options
that are always sent in the DHCP protocol should not
be specified
here, except that the client can specify a
dhcp-lease-time option
other than the default requested lease time, which
is two hours.
The other obvious use for this statement is to send
information
to the server that will allow it to differentiate
between this
client and other clients or kinds of clients.

In some cases, a client may receive option data from the
server which is
not really appropriate for that client, or may not receive
information
that it needs, and for which a useful default value exists.
It may also
receive information which is useful, but which needs to be
supplemented
with local information. To handle these needs, several option modifiers
are available.
default { [option declaration] [, ... option declaration] }
If for some set of options the client should use the
value supplied
by the server, but needs to use some default
value if no
value was supplied by the server, these values can
be defined in
the default statement.
supersede { [option declaration] [, ... optiondeclaration]
}
If for some set of options the client should always
use its own
value rather than any value supplied by the server,
these values
can be defined in the supersede statement.
prepend { [option declaration] [, ... option declaration] }
If for some set of options the client should use a
value you supply,
and then use the values supplied by the server,
if any,
these values can be defined in the prepend statement. The
prepend statement can only be used for options which
allow more
than one value to be given. This restriction is not
enforced -
if violated, the results are unpredictable.
append { [option declaration] [, ... option declaration] }
If for some set of options the client should first
use the values
supplied by the server, if any, and then use values
you supply,
these values can be defined in the append statement.
The append
statement can only be used for options which allow
more than one
value to be given. This restriction is not enforced
- if you ignore
it, the behaviour will be unpredictable.

The lease declaration:
lease { lease-declaration [... lease-declaration] }
The DHCP client may decide after some period of time (see
PROTOCOLTIMING) that it is not going to succeed in contacting a
server. At that
time, it consults its own database of old leases and tests
each one that
has not yet timed out by pinging the listed router for that
lease to see
if that lease could work. It is possible to define one or
more fixed
leases in the client configuration file for networks where
there is no
DHCP or BOOTP service, so that the client can still automatically configure
its address. This is done with the lease statement.
NOTE: the lease statement is also used in the
dhclient.leases file in order
to record leases that have been received from DHCP
servers. Some of
the syntax for leases as described below is only needed in
the
dhclient.leases file. Such syntax is documented here for
completeness.
A lease statement consists of the lease keyword, followed by
a left curly
brace, followed by one or more lease declaration statements,
followed by
a right curly brace. The following lease declarations are
possible:
bootp; The bootp statement is used to indicate that the
lease was acquired
using the BOOTP protocol rather than the DHCP
protocol.
It is never necessary to specify this in the client
configuration
file. The client uses this syntax in its lease
database file.
interface"string";
The interface lease statement is used to indicate
the interface
on which the lease is valid. If set, this lease
will only be
tried on a particular interface. When the client
receives a
lease from a server, it always records the interface
number on
which it received that lease. If predefined leases
are specified
in the dhclient.conf file, the interface should also
be specified,
although this is not required.
fixed-addressip-address;
The fixed-address statement is used to set the IP
address of a
particular lease. This is required for all lease
statements.
The IP address must be specified as a dotted quad
(e.g.,
12.34.56.78).
filename"string";
The filename statement specifies the name of the
boot filename to
use. This is not used by the standard client configuration
script, but is included for completeness.
server-name"string";
The server-name statement specifies the name of the
boot server
name to use. This is also not used by the standard
client configuration
script.
optionoption-declaration;
The option statement is used to specify the value of
an option
supplied by the server, or, in the case of predefined leases declared
in dhclient.conf, the value that the user
wishes the
client configuration script to use if the predefined
lease is
used.
script"script-name";
The script statement is used to specify the pathname
of the DHCP
client configuration script. This script is used by
the DHCP
client to set each interface's initial configuration
prior to requesting
an address, to test the address once it has
been offered,
and to set the interface's final configuration once a
lease has been acquired. If no lease is acquired,
the script is
used to test predefined leases, if any, and also
called once if
no valid lease can be identified. For more information, see
dhclient.leases(5).
medium"media setup";
The medium statement can be used on systems where
network interfaces
cannot automatically determine the type of
network to which
they are connected. The media setup string is a
system-dependent
parameter which is passed to the DHCP client configuration script
when initializing the interface. On UNIX and UNIXlike systems,
the argument is passed on the ifconfig command line
when configuring
the interface.
The DHCP client automatically declares this parameter if it used
a media type (see the media statement) when configuring the interface
in order to obtain a lease. This statement
should be
used in predefined leases only if the network interface requires
media type configuration.
renewdate;
rebinddate;
expiredate;
The renew statement defines the time at which the
DHCP client
should begin trying to contact its server to renew a
lease that
it is using. The rebind statement defines the time
at which the
DHCP client should begin to try to contact any DHCP
server in order
to renew its lease. The expire statement defines the time at
which the DHCP client must stop using a lease if it
has not been
able to contact a server in order to renew it.
These declarations are automatically set in leases acquired
by the DHCP
client, but must also be configured in predefined leases - a
predefined
lease whose expiry time has passed will not be used by the
DHCP client.
Dates are specified as follows:
<weekday> <year>/<month>/<day> <hour>:<minute>:<second>
The weekday is present to make it easy for a human to tell
when a lease
expires - it's specified as a number from zero to six, with
zero being
Sunday. When declaring a predefined lease, it can always be
specified as
zero. The year is specified with the century, so it should
generally be
four digits except for really long leases. The month is
specified as a
number starting with 1 for January. The day of the month is
likewise
specified starting with 1. The hour is a number between 0
and 23, the
minute a number between 0 and 69, and the second also a number between 0
and 69.

alias { declarations ... }
Some DHCP clients running TCP/IP roaming protocols may require that in
addition to the lease they may acquire via DHCP, their interface also be
configured with a predefined IP alias so that they can have
a permanent
IP address even while roaming. The Internet Software Consortium DHCP
client doesn't support roaming with fixed addresses directly, but in order
to facilitate such experimentation, the DHCP client can
be set up to
configure an IP alias using the alias declaration.
The alias declaration resembles a lease declaration, except
that options
other than the subnet-mask option are ignored by the standard client configuration
script, and expiry times are ignored. A typical
alias declaration
includes an interface declaration, a fixed-address
declaration for
the IP alias address, and a subnet-mask option declaration.
A medium
statement should never be included in an alias declaration.

rejectip-address;
The reject statement causes the DHCP client to reject offers from
servers who use the specified address as a server
identifier.
This can be used to avoid being configured by rogue
or misconfigured
DHCP servers, although it should be a last resort - better
to track down the bad DHCP server and fix it.
interface"name" { declarations ... }
A client with more than one network interface may
require different
behaviour depending on which interface is being
configured.
All timing parameters and declarations other than
lease and alias
declarations can be enclosed in an interface declaration, and
those parameters will then be used only for the interface that
matches the specified name. Interfaces for which
there is no interface
declaration will use the parameters declared
outside of
any interface declaration, or the default settings.
media"media setup" [, "media setup", ...];
The media statement defines one or more media configuration parameters
which may be tried while attempting to acquire an IP address.
The DHCP client will cycle through each media setup
string on the list, configuring the interface using
that setup
and attempting to boot, and then trying the next
one. This can
be used for network interfaces which aren't capable
of sensing
the media type unaided - whichever media type succeeds in getting
a request to the server and hearing the reply is
probably right
(no guarantees).
The media setup is only used for the initial phase
of address acquisition
(the DHCPDISCOVER and DHCPOFFER packets).
Once an address
has been acquired, the DHCP client will record
it in its
lease database and will record the media type used
to acquire the
address. Whenever the client tries to renew the
lease, it will
use that same media type. The lease must expire before the
client will go back to cycling through media types.